CS 340

UML Class Diagrams

A model is an abstraction of a system, specifying the modeled system from a certain viewpoint

and at a certain level of abstraction

Complexity and Modeling • Modeling is a well-accepted engineering technique

for dealing with complexity • Models let us focus on one view of the system while

ignoring irrelevant details (a simplification of reality) – Example: Plumbers only care about plumbing, so we give

them plumbing diagrams; electricians only care about electrical work, so we give them electrical diagrams, etc.

• Models can be created at different levels of abstraction – 10,000 ft. view vs. 10 ft. view – High-level: sketch of the building's exterior for the customer – Low-level: detailed electrical plan for the electrician

Software Modeling

• Why do we create models of software systems?

• Models help us visualize and understand the system we're building

• Models are used to document the design of the system and communicate it to others

• Models serve as a guide during system construction

UML

• Unified Modeling Language • History • Models

– Data – Behavior – Interaction – Packaging

Levels of Models • Conceptual Model – Problem Understanding • Specification/Requirement Document – Solutions • High-Level Design Model

– Systems, Packages, Classes – Interactions, Interfaces

• Low-Level Design Model – Class Conceptual Model – May be better done with Class documentation such

as Javadoc • Implementation

– Should be done in code not UML

UML Models • Structural/Data

– Class Diagram – Object Diagram

• Behavior – Use Case Diagram – State Diagram – Activity Diagram

• Interaction – Communication Diagram – Sequence Diagram

• Physical Diagram – Component Diagram – Deployment Diagrams

Classes At the Conceptual Level

A Class – typically named with a common noun. At the conceptual level the class is a set of objects with common properties.

Mother

Classes At the Design Level A Class – typically named with a common noun. At the design level a Class is a Type. Visibility constraints (optional) : + (public), - (private), # (protected), / (derived), _ (static), ~ (package). We may add cardinality constraints to an attribute. In this case it means that a Mother has 1 or more names. If the cardinality constraint is omitted it is assumed to be the cardinality constraint “1”. For some situations we may omit the Method Section or the Attribute Section.

Mother

- name : Name [1.. *] - age : int

+ print() + equals(Object)

Attribute Section

Method Section

Objects An Object – notice the name is underlined. While some find Objects useful in the Class diagram others do not.

Clara

Associations An association can be thought of as a declarative sentence. The name in the association is optional but suggested. Typical association constraints are 1, 0..1, *, 1..*, n..m. UML 1.0 supported a notation for an arbitrary set of non-negative integers (e.g. 2:4, 6,12 meaning the set {2,3,4,6,12} Most associations are binary.

Child Mother 1 *

has

Associations Continued • Roles

– Although this example has 2 roles, there may be only 1, or most

often 0 roles on an association. – Roles can be viewed as Specializations

• While not all versions of UML support the same annotations, you may find some when reading UML diagrams.

Male Female 1 1 is married to

Husband Wife

Mother Child 1..* 1

has {ordered}

Directed Associations • Should only be used as a design construct

– Means that we can access an Address via a Person, but, given an Address we can’t determine the Person associated with it.

– If the association constraint on the opposite side of the navigability arrow is a 1, then this is equivalent to

– Notice the cardinality constraint to the right of the attribute.

Person Address 1..* 1

has

Person

address : Address [1..*]

Aggregation/Composition • Aggregation

– Can be read as Person is subpart of a Company – Can be transitive but acyclic. – Some people prefer to just use associations. – Can use “comb” representation of there are different types of subparts.

• Composition

– Can be read as “A Car is composed of 1 or more Car Parts”. – A much stronger form of Aggregation. Some suggest it implies a Car Part

cannot exist unless it is part of a car. – Notice there is no association constraint next to the black diamond

because it is always 1.

Company Person 1..*

0..* Employee

Car Car Part 1..*

Generalization/Specialization – For conceptual modeling every Faculty Member, Staff Member, and

Student is a BYU Person. “Is A” Semantics. That is, subset semantics. In this case the set, Student, is a subset of the set BYU Person.

– At the Design level it represents “subtype” or “inheritance” semantics. – A generalization/specialization often has a single specialization. – Though not standard UML, I may include the following annotations:

{union}, {disjoint}, and {partition}.

BYU Person

Faculty Member

Staff Member Student

General Constraints/Notes

• General Constraints – {A Father must be a male Person} – I often omit the braces. – Can be expressed formally in OCL, a type of first-

order logic. • Notes

– I use text such as

NOTE: A Note or Note: Information

A Note Family History

Dependency

• Used in design models. Especially good for representing dependencies that aren’t obvious, such as assumptions.

• There is no notation for indicating why Person depends on Address. I use notes to add the information.

Person Address

Association Class • Useful when thinking of an association as an object. • Could use an n-ary relation.

Husband Wife 1..* 1..*

married

Marriage Information

date : Date temple: Temple

N-ary Associations

• In UML the association constraints are binary functional dependencies. • They tried to extend that idea to n-ary associations. It didn’t work well. • Here the 4 association constraints mean

– For a given Husband, Wife, and Temple in the association there are 1 or more Dates. – For a given Husband, Wife, and Date in the association there is 1 and only 1 Temple. – For a given Husband, Date and Temple in the association there is 1 and only 1 Wife. – For a given Wife, Date, and Temple in the association there is 1 and only 1 Husband.

• Because this is awkward it is not used as much as it could be. • If we interpret the constraints as participation constraints (see ORM, and

OSA) then the semantics are much easier to understand.

Husband Wife 1 1

Date

Temple

1..*

A Husband married a Wife on a given Date at a particular Temple

1

Class Diagram Example

+floor : float+direction : Direction

Elevator

+NONE+UP+DOWN

«enumeration»Direction

Building

1 1..*

+number : intFloor

1

1..*

+Push()+illuminated : bool

Button

+floorNumber : intElevator Button

+direction : DirectionFloor Button

1 2

1

1..*

Person

* 0..1

*

0..1

{An Elevator has a button foreach Floor}

{A Person is always associated with a Flooror an Elevator, but never both at the same time}

{A Floor has one Down button andone Up button}

Class Diagram Example

Source: Booch et. al., The UML User Guide

+name : stringCompany

name : stringDepartment

1

1..*

address : stringvoice : string

Office

11..*

* *

Is Located At0..1

*

GetContactInformation() : ContactInformationGetPayrollInformation() : PayrollInformation

name : stringemployeeID : inttitle : string

Person

address : stringContactInformation

taxID : stringsalary : float

PayrollInformation

Headquarters

*

member1..*

*

manager1